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Date: Fri, 30 Jan 2009 12:41:02 -0800
From: Mandeep Singh Baines <msb@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
       rientjes@google.com, mbligh@google.com, thockin@google.com,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v4] softlockup: remove hung_task_check_count
Message-ID: <20090130204102.GR22298@google.com>
References: <1232991701.4863.222.camel@laptop> <20090127003055.GA21269@google.com> <20090127132626.GH23121@elte.hu> <20090127184851.GD22298@google.com> <1233131100.10992.43.camel@laptop> <20090129014245.GM22298@google.com>
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Mandeep Singh Baines (msb@google.com) wrote:
> Peter Zijlstra (peterz@infradead.org) wrote:
> > 
> > Why break out at all? Are you that worried about khungtaskd introducing
> > latencies?
> 
> Yes, I was worried about disabling preemption for an unbounded amount of
> time.
> 
> > Is using preemptible RCU an option for you?
> > 
> 
> I had not even considered that. To be honest, I had not even heard of it
> till now. So I spent another morning at LWN grokking preemptible RCU;)
> 
> I think it can work. I'm a little worried about the OOM risk. It could take
> a really long time to iterate over the task list. A lot of pending kfree()s
> could build up in that time.
> 

I misunderstood preemptible RCU. I assumed it was a new API but its not. So
I don't think preemptible RCU is an option since it would force a dependency
on CONFIG_PREEMPT_RCU.

I'm going to break up this patch in two. One patch for converting to rcu.
A second patch which will support checking all tasks. To support checking
all tasks I reverted back to a design similar to Fr?d?ric original proposal.

I'll send the patches out right after this email.

[PATCH 1/2] softlockup: convert read_lock in hung_task to rcu_read_lock
[PATCH 2/2] softlockup: check all tasks in hung_task

Regards,
Mandeep
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